This invention relates generally to deflection compensation systems for use in digital television (TV) receivers.
In a cathode ray tube (CRT) raster scan system the electron beam is horizontally and vertically deflected across the phosphor target on the face of the CRT by the magnetic field produced by a set of deflection windings situated on the CRT neck. In conventional reaction-scan systems the high voltage required for operation of the CRT is derived from the horizontal deflection system and is very sensitive to both changes in beam current and changes in B+ operating potential. Also, the size of the raster varies as a function of the high voltage applied. Should the high voltage decrease, for example, the electron beam is more readily deflected and the raster size increases. Thus, for a given scan or deflection voltage the raster size varies inversely with the high voltage applied. In most systems the scan voltage is closely related to the B+ operating potential, which tends to make such deflection systems self-compensating. Thus, an increase in beam current will give rise to a corresponding decrease in high voltage, which would normally cause an increase in the raster size, except for the fact that the scan voltage will decrease correspondingly because of the drop in B+ operating potential. Thus the sum of the effects tends to keep the raster size relatively constant and raster size changes with beam current or B+ changes is not a major problem in conventional television receivers.
With the advent of so-called digital television receivers, where analog functions are replaced by digital circuitry, the ramp-shaped vertical scan current is developed from a pulse width modulated (PWM) signal that consists of a series of fixed amplitude pulses of differing durations. Integration of the PWM signal results in an analog ramp current for producing the vertical deflection of the CRT beam. This PWM signal is, for all practical purposes, insensitive to changes in B+ operating potential. Consequently, in these receivers, variations in B+ can produce annoying changes in the height of the raster unless some type of compensation is employed.